1. Field of the Invention
The present invention relates to a semiconductor device having plural gate insulating films with different film thicknesses, from each other, and a manufacturing method therefor.
2. Description of the Related Art
In the prior art, a method of forming a multioxide device includes forming gate insulating films with different film thicknesses on the same chip. When transistors having different purposes are formed on the same chip, it is necessary to design respective parameters, such as the film thickness of gate oxide film, for each transistor to satisfy performance requirements for each transistor. For example, the film thickness of the gate oxide film must be reduced with reduction of power source voltage in a transistor required to operate at a high speed, while the film thickness of the gate oxide film must be increased with a requirement of comparatively high power source voltage in a transistor used for interface with the outside of the chip.
A prior art multioxide forming method will be described with reference to FIGS. 1 to 6. The left sides of FIGS. 1-6 show a region where a thin oxide film is formed, while the right sides of FIGS. 1-6 show a region where a thick oxide film is formed. First, as illustrated in FIG. 2, a first gate oxidation is performed to form the thick gate oxide film 102 on a silicon substrate 101. Then, as illustrated in FIG. 3, a resist 103 is formed at a right side region where the thick oxide film is to be formed. Then, as also illustrated in FIG. 3, etching is performed by using hydrofluoric-acid solution to remove the left side gate oxide film at a left side region where the thin oxide film is to be formed. Thereafter, the resist is stripped to produce the structure illustrated in FIG. 4. Preprocessing of cleaning and dilute hydrofluoric-acid (DIIF) treatment is then performed to produce the structure illustrated in FIG. 5 on the silicon surface at the left side region where the thin oxide film is to be formed after the left side gate oxide film has been etched. Then, the second gate oxidation is performed to form the thin gate oxide film 105 is formed at the left side region, as illustrated in FIG. 6.
In the resulting prior art structure, the thick oxide film which requires a clean state is contaminated with the resist 103 formed on it. However, since the thick oxide film is cleaned at the later cleaning process, and above all, the multioxide can be easily formed, the method is widely used.
Further, a second prior art method includes formation of multioxide by oxidation speed control by ion implantation. The multioxide formation method by ion-implantation forms gate oxide films with different film thicknesses by pattern forming a resist on a silicon substrate, selectively implanting ions for thermal oxidation speed control, and performing thermal oxidation after stripping the resist.
However, the second prior art method has various problems and disadvantages. For example, gate oxide film films with a large film thickness difference cannot be obtained. However, the second prior art method prevents contamination of gate oxide film by the resist coating, and the process steps can thus be reduced.
The prior art film thickness of multioxide has been further reduced as the micromininaturization and techniques for high-speed processing have further advanced. For example, the required film thickness of gate oxide film at a core region where high-speed processing is less than 2.0 nm. On the formation of a gate oxide film, it is necessary to clean the surface to eliminate contamination of the resist before forming the gate oxide film by thermal oxidation. The prior art gate oxidation is performed immediately after the cleaning, using any combination of aqueous ammonia hydrogen peroxide, aqueous hydrochloric acid hydrogen peroxide and aqueous sulfuric acid hydrogen peroxide.
However, when the gate oxide film is thinner than 2.0 nm, it is difficult to form the gate oxide film immediately after the cleaning, because the above-described cleaning forms an oxide film having a thickness of about 1.0 nm (i.e., a natural oxide film or chemical oxide film) on the silicon surface during the cleaning. Although the final film thickness of the gate oxide film is 2.0 nm or less, thermal oxidation is performed after the 1.0 nm natural oxide film is formed by cleaning, which means the half or more in thickness of the film that should have formed by the thermal oxidation is formed by the natural oxide film. This natural oxide film may significantly degrade the reliability of its insulation, because of the low density of the natural oxide. In addition, the condition of formation of the natural oxide film depends on cleaning conditions, and its film thickness is difficult to control. Variation in the final film thickness of the gate oxide film results. Therefore, when the gate oxide film having a film thickness of 2.0 nm or less is necessary, natural oxide film grown in the cleaning must be removed before forming a gate oxide film.
The gate oxidation after the removal of natural oxide film causes no problem upon formation of transistors having the same gate oxide films in their thicknesses. However, when multioxide is formed, a problem occurs in that the gate oxide film in a thick-film region in the multioxide is reduced with the natural oxide film formed at a thin-film formation region.
In the prior art, an oxide film is removed by a hydrofluoric-acid base chemical agent. Because the etching is performed for a while after the silicon substrate is exposed at the thin gate oxide film region to remove as much of the natural oxide film as possible, the etching also progresses in the oxide film at the thick gate oxide film region. Therefore, more of the gate oxide film at the thick gate oxide film region is etched than expected.
As a result, various problems and disadvantages result. For example, the reproducibility of film thickness at the thick film region is remarkably degraded when etching reproducibility is poor. Additionally, a conductive portion called a pin hole 104 forms in the oxide film, which degrades the insulation, since the hydrofluoric-acid solution selectively etches a weak portion of the thick-gate oxide film. While an oxide film appears to have uniform thickness and seems to be uniformly etched in its film thickness direction by etchant, the oxide film actually often has a nonuniform thickness at a microscopic level (e.g., in atomic order). Thus, the prior art oxide film has weak portions. Additionally, the hydrofluoric-acid solution selectively attacks the weak portions to overetch the films of those portions more than other (e.g., peripheral) portions. Therefore, the local thin film region is made in the thick film region. The pin hole 104 is an extreme example of an overetched portion, and the insulation is remarkably degraded at one portion, and the reliability in the insulation of the entire thick-film gate oxide film is thus degraded.
It is an object of the present invention to provide a semiconductor device, having a thick film region not damaged by the hydrofluoric-acid treatment, and a manufacturing method therefor predicated on hydrofluoric-acid treatment.
It is another object of the present invention to perform resist coating and selective etching in a region where a thin oxide film is to be formed, different from the multioxide formation method by ion-implantation.
A semiconductor device in an embodiment of the present invention comprising, a substrate that has a first region and a second region, a first film formed on said first region of said substrate and a second film formed on said second region of said substrate, wherein said second film is thinner than said first film, and said first film is resistant to etching during removal of a natural oxide.
To accomplish at least the above objects, a preferred embodiment provides a method of manufacturing a semiconductor device, comprising (a) forming a first film on a first region of a substrate and (b) forming a second film on a second region, wherein said first film is substantially resistant to etching, and a natural oxide film is removed from said second region before forming said second film on said second region.
A further preferred embodiment provides a method of manufacturing a semiconductor device, comprising (a) forming a first film on a first region of a substrate, (b) forming a second film on a second region, wherein said first film is substantially resistant to etching, and a natural oxide film is removed from said second region before forming said second film on said second region; and (c) forming a third film which is thinner than said second film on a third region of said substrate, wherein said third film is substantially resistant to etching a second natural oxide film on said third region of said substrate before forming said third film.
Another preferred embodiment provides method of manufacturing a semiconductor device, comprising (a) forming a first gate silicon oxide layer on said substrate in said first region and said second region, (b) forming a cover layer on said first gate silicon oxide film on said first region and said second region, and (c) forming a resist on said first film in said first region, and removing said first film from said second region by using said resist as a mask. The method also comprises d) forming a second film on a second region, (e) removing said first film from said second region, using said resist as a mask; and (f) removing said resist from said first region to form said first film in said first region. The method further comprises (g) removing contamination from said second region, said contamination being caused by said resist, wherein said natural oxide film is formed on said second portion at said removing contamination step and (h) removing said natural oxide film from said second region by using said first film as a mask for said first region, wherein said first film is substantially resistant to etching, and a natural oxide film is removed from said second region before forming said second film on said second region.
According to the embodiment of the present invention, the thick gate oxide film is not almost etched by the stripping process, the cleaning process and the hydrofluoric-acid treatment process performed at the thin gate oxide film region because the oxinitride film having higher etching resistance than the thin gate oxide film is formed on the surface of the thick gate oxide film. Thus precise film thickness control is realized.